1. Field of the Invention
The present invention relates to a semiconductor photodetector, and more particularly, to a photodetector which is useful in detecting light having a wavelength of ultraviolet and blue ranges.
2. Description of the Related Art
In a photodetector as a semiconductor device, an ultraviolet detector is being widely used in military, environmental, and industrial fields, for example a flame monitor, a water purification system, money counting and fake detection machines, a sunlight exposure meter, photochemical phenomena detection, etc.
As the most widely used material for a semiconductor, silicon has an energy gap of 1.1 eV and reacts to visible light, so that it is difficult to selectively detect only light having a wavelength of an ultraviolet range. Further, as a representative photodetector, a PN or PIN junction diode absorbs most of the light of short wave length in the vicinity of a surface thereof, so that the created carriers are recombined and do not substantially contribute to the electrical signal. For this reason, the ultraviolet detector mainly includes a material such as gallium nitride (GaN), silicon carbide (SiC), etc., which has a relatively large energy gap, and is generally made into a schottky diode to reduce a loss due to the recombination.
Generally, the semiconductor photodetector is made in forms of a PN junction diode, a PIN junction diode, a schottky diode, a metal oxide semiconductor (MOS) capacitor, etc., wherein a depletion region of the semiconductor is employed as a light detection region of the photodetector.
In the conventional photodetector using the PIN diode, a substrate having an N− intrinsic layer grown thereon by an epitaxial process is used. On the substrate are formed a P+ diffusion region and a P guardring region by an ion implantation process. Then, a dielectric layer is formed, and the light entrance window is opened. That is, the light entrance window includes the P+ diffusion region.
The foregoing PIN diode fabricated using a silicon wafer as described above is effective in detecting visible light or near infrared. However, the PIN diode has a limit to detect the light having a wavelength of ultraviolet and blue ranges because most of the light is absorbed at the surface of the semiconductor, and generated carriers are recombine. Further, the PIN diode is inadequate to be employed as the photodetector for selectively detecting the ultraviolet light.
On the other hand, if a semiconductor material such as silicon carbide (SiC), gallium nitride (GaN), etc. is used instead of silicon, it is possible to filter relatively long wavelength light such as visible light, infrared light, etc., but there is a loss due to absorption and recombination at the surface of the semiconductor. In the case of silicon carbide (SiC), manufacturing process has to be performed separately from a universal silicon process because of carbon contamination, and is incompatible with the silicon CMOS process. In the case of a compound semiconductor such as gallium nitride (GaN), etc., the wafer is expensive and the process is complicated.
Meanwhile, the schottky diode made of the silicon wafer allows the photodetector to be improved in detecting the light having the wavelength of the ultraviolet range, but causes the light to be reflected from and absorbed in a schottky contact metal thereof. Likewise, a schottky diode process is incompatible with the silicon CMOS process. Further, in the case of the schottky diode using the compound semiconductor such as GaN, SiC, etc., the wafer is expensive and the process is complicated.